The present invention generally relates to the use of voltage variable materials for the protection of electronic components against electrical overstress (EOS) transients.
There is an increased demand for materials and electrical components which can protect electronic circuits from EOS transients which produce high electric fields and usually high peak powers capable of destroying circuits or the highly sensitive electrical components in the circuits, rendering the circuits and the components non-functional, either temporarily or permanently. The EOS transient can include transient voltage or current conditions capable of interrupting circuit operation or destroying the circuit outright. Particularly, EOS transients may arise, for example, from an electromagnetic pulse, an electrostatic discharge, lightening, or be induced by the operation of other electronic or electrical components. Such transients may rise to their maximum amplitudes in microsecond to subnanosecond time frames and may be repetitive in nature. A typical waveform of an electrical overstress transient is illustrated in FIG. 1. The peak amplitude of the electrostatic discharge (ESD) transient wave may exceed 25,000 volts with currents of more than 100 amperes. There exist several standards which define the waveform of the EOS transient. These include IEC 1000-4-2, ANSI guidelines on ESD (ANSI C63.16), DO-160, and FAA-10-136. There also exist military standards, such as MIL STD 461 and MIL STD 883 part 3015.
Materials for the protection against EOS transients (EOS materials) are designed to respond essentially instantaneously (i.e., ideally before the transient wave reaches its peak) to reduce the transmitted voltage to a much lower value and clamp the voltage at the lower value for the duration of the EOS transient. EOS materials are characterized by high electrical resistance values at low or normal operating voltages and currents. In response to an EOS transient, the material switches essentially instantaneously to a low electrical resistance value. When the EOS threat has been mitigated, these materials return to their high resistance value. These materials are capable of repeated switching between the high and low resistance states, allowing circuit protection against multiple EOS events. EOS materials are also capable of recovering essentially instantaneously to their original high resistance value upon termination of the EOS transient. For purposes of this application, the high resistance state will be referred to as the xe2x80x9coff-statexe2x80x9d and the low resistance state will be referred to as the xe2x80x9con-state.xe2x80x9d
FIG. 2 illustrates a typical electrical resistance versus d.c. voltage relationship for EOS materials. Circuit components including EOS materials can shunt a portion of the excessive voltage or current due to the EOS transient to ground, thus, protecting the electrical circuit and its components. The major portion of the threat transient is reflected back towards the source of the threat. The reflected wave is either attenuated by the source, radiated away, or re-directed back to the surge protection device which responds with each return pulse until the threat energy is reduced to safe levels.
The present invention relates to the application of voltage variable materials to numerous different configurations of electrical devices to provide protection from EOS transients. Pursuant to the present invention, any voltage variable material can be used.
In a first embodiment of the present invention, an electrical device (e.g., a chip comprising an integrated circuit) comprises at least one, and preferably, a plurality of electrical leads, a ground plane and a layer of voltage variable material. Each electrical lead is spaced apart from the other electrical leads and the ground plane. The layer of voltage variable material is located on the spacing between the electrical leads and the ground plane, physically connecting the electrical leads and the ground plane. In a preferred embodiment, the voltage variable fills the spacing between the electrical leads and the ground plane.
The device can be manufactured by providing a lead frame having a plurality of electrical leads and a ground plane. There is a predetermined spacing, A, between each electrical lead and a predetermined spacing, B, between each lead and the ground plane. A voltage variable material is applied to the lead frame such that the material is located in the spacing, A, between the leads and the spacing, B, between each lead and the ground plane. The voltage variable material physically connects the plurality of leads to the ground plane. In a preferred embodiment, the material fills the spacing A and B. A die or integrated circuit is mounted on the ground plane. The leads are then electrically connected to the integrated circuit. The plurality of electrical leads, ground plane and die are encapsulated in a protective coating leaving a portion of the plurality of leads exposed. The exposed portion of the leads are then formed into connector pins. Advantages of such a device and method are: (1) the need for a separate substrate to accommodate the voltage variable material, electrical leads, ground plane and die is eliminated; (2) protection from EOS transients is built into the existing structure of the electrical device, thus, conserving real estate on a printed circuit board; and (3) the voltage variable material can be applied to a variety of device configurations.
In another embodiment of the present invention, the voltage variable material forms a tape upon which a circuit is built up to form the electrical device. Starting with a layer of voltage variable material, a first electrode is attached to a first side of the layer and a second electrode is attached to a second side of the layer. The electrodes can be a metal foil or a metal layer attached by any conventional method (e.g., lamination, electroless plating, electrolytic plating, vapor deposition). The electrodes are then processed by masking/etching or photo lithographic methods (as disclosed in U.S. Pat. No. 5,699,607 and incorporated herein by reference) to form a laminate comprised of a ground plane and a plurality of electrical leads on the layer of voltage variable material. The layer of voltage variable material is then cured and pressure is applied to the laminate to form the final integrated tape device. In order to provide mechanical stability to the device during the processing of the electrodes, the voltage variable material may be cured prior to the step of processing the electrodes.
In yet another embodiment of the present invention, a hybrid tape of voltage variable material is simply bonded to an existing lead frame to provide protection against EOS transients. The lead frame includes a plurality of electrical leads electrically connected to the die having a circuit integrated therein. The die is attached to a conductive die connector plate. The hybrid tape is comprised of a layer of voltage variable material with a first conductive layer disposed on one side thereof. The other side of the layer of voltage variable material is bonded to the lead frame, physically connecting the plurality of electrical leads to the die connector plate. The plurality of electrical leads are then trimmed from the lead frame to produce the electrical device.
In a further embodiment of the present invention, a layer of voltage variable material has a conductive adhesive applied to a first side. The adhesive has a anisotropic voltage breakdown. A conductive layer (e.g., metal foil or electroless layer of copper) is applied to a second side of the layer of voltage variable material to form a tape. Preferably, the conductive adhesive collapses to a very thin layer when bonded to an object (e.g., a lead frame or pin-style electrical connection of an electrical device to a trace on a printed circuit board).
In still another embodiment of the present invention, the electrical device comprises a hollow conductive tube having a layer of voltage variable material disposed on the outer surface thereof. The electrical device is especially well suited for providing protection between the electrical leads of a connector housing. The device is placed between a first set of plurality of electrical leads and a second set of a plurality of electrical leads, with the layer of voltage variable material contacting both the first and second sets of plurality of leads and the conductive tube being electrically grounded.
In another embodiment of the present invention, an integrated circuit is encapsulated in a protective housing comprised of a voltage variable material. The protective housing bonds the individual components of the circuit together, protects the integrated circuit from undesired external influences (mechanical and electrical), and electrically protects the integrated circuit from EOS transients.
In a final embodiment of the present invention, the housing of an electrical connector is composed of a voltage variable material. The material is applied between the electrical connectors to which an EOS transient may be applied and the ground reference of the electrical connectors.
Additional advantages and features of the present invention will become apparent upon reading the following detailed description of the presently preferred embodiments, appended claims, and upon reference to the attached drawings.